The invention relates to a series-parallel-series digital system with at least one storage function, comprising a first number of n (n&gt;1) series-parallel-series digital units, each of which comprises a second number of k(k&gt;1) elements, a further number r (1.ltoreq.r.ltoreq.k) thereof being defective in each unit, each digital unit comprising a serial input and a serial output for connection to a bus which comprises at least p(p&gt;1) channels for the serial transport of a data stream of digital bits via each channel.
Such a series-parallel-series digital system is known from the article "Multiplexed partial-good chip scheme employing defective loops as selectors for all-good chips" by F. J. Aichelmann, Jr., published in I.B.M. T.D.B., Vol. 22, No. 1, June 1979, pp. 138-139. The series-parallel-series digital system described therein is a series-parallel-series shift register system in which each memory unit comprises at least one defective memory element. Consequently, it is not possible to store information in said defective memory element in a reliable manner. In order to enable reliable information storage, however, the system includes an additional memory unit which comprises only memory elements which are not defective. When such a defective memory element is addressed, the relevant address is converted into an address for the additional memory unit in order to read or write information from or into the additional memory unit.
However, the total capacity of the system will not be required in given circumstances or for given applications. This is the case, for example when not all channels of the bus are used, for example in digital television where only 7 of 8 bus channels are used. It may also be that operations need be performed only on words which are transported via given channels of the bus, or that the supply of information requires only a part of the capacity of the system. It may also be that, for example when multibit words are used, unreliability occurs in the least-significant bit due to a defective element which, however, has no adverse effects on the further processing of the relevant word. For this type of circumstances and applications it is feasible to use only digital units which comprise one or sometimes more than one defective element and which are cheaper than all-good digital units.